1. Field of the Invention
This invention relates to the field of superplastically formable aluminum alloys, and more particularly to means for imparting superplastic formability to aluminum alloys with relatively lower magnesium concentrations, i.e., those with about 4 wt. % magnesium or less. The invention further relates to an improved sheet product made from said alloys, said sheet product having improved corrosion resistance thereby making it more suitable for use in numerous applications, especially those in the automotive field.
2. Technology Review
Numerous approaches are known for enhancing superplastic formability. Some are directed to manipulations in the superplastic forming operation to enhance said operation or alleviate problems associated with it largely by controlling the flow of metal during forming. Representative examples of such manipulations are shown in U.S. Pat. Nos. 3,997,369, 4,045,986, 4,181,000 and 4,516,419. Another approach is directed to the metal to be superplastically formed. It has long been recognized that fine grain size enhances forming operations, including superplastic forming. Some efforts to achieve fine grain size are shown in U.S. Pat. Nos. 3,847,681 and 4,092,181. More recently, U.S. Pat. No. 5,055,257 taught adding scandium and zirconium to certain aluminum alloys for achieving SPF properties in 7050-type alloy.
There are several known ways for achieving superplastic formability in aluminum alloys with relatively higher magnesium contents, i.e. generally above 4 wt. % Mg. For Al sheet products containing about 4.5 wt. % or higher Mg, even up to about 10% Mg, the Zr, Cr and/or Mn dispersoids that are usually present develop superplastic forming (SPF) capabilities with elongations of about 400-550% at moderately fast strain rates of about 2.times.10.sup.-3 /sec or more when subjected to certain thermomechanical process (or "TMP") combinations. The latter rates are similar to the relatively fast strain rates available for a commercial SPF aluminum alloy sold by Superform under the mark Supral.RTM..
Al alloys with magnesium contents of about 3 wt. % have superior corrosion resistance compared to their higher Mg (4.5 wt. % and above) counterparts, thus making lower magnesium-containing, aluminum alloys attractive for many automotive part applications, especially when such parts can be made by superplastic forming ("SPF") to achieve part consolidation. When subjected to identical TMP conditions as those described above for higher Mg alloys, however, an Al-3% Mg alloy resulted in a maximum elongation of only about 208%. There is a current need to develop an SPF Al--Mg for possible automotive parts consolidations. Most efforts have centered around 4.5% Mg compositions because of automakers' and researchers' past familiarity with 5083 and 5182 alloy performance, an SPF alloy with only about 3% Mg would be preferred in spite of its somewhat reduced strength, since such lower Mg alloys are less susceptible to intergranular corrosion when exposed to paint bake temperatures unlike their Al-4.5 Mg counterparts. Such a development could provide a differentiated product for possible use in both inner part and outer panel automotive applications. Said products would have superior corrosion resistance coupled with high SPF formability. This invention addresses recent efforts to achieve good SPF elongations, in excess of about 400%, and more preferably 500% or higher, for about 88% cold rolled sheet at about 1050.degree. F. using a moderately high strain rate of about 0.001/sec. The results obtained herein compare favorably with SPF results reported in the literature for the 4.5% Mg-based 5083 and 5182 aluminum alloys favored by today's automotive manufacturers and designers. It is believed that the same procedures described below for a new Al-3% Mg SPF product could also enhance the performance of higher Mg-containing alloys, including Al-4.5% Mg alloys.